1. Field of the Invention
This invention relates to shift register control in general and more particularly to a control system for dynamic recirculating shift register memories which permits random access to the contents of the shift register.
2. Description of the Prior Art (Prior Art Statement)
The dynamic recirculating shift register memory was introduced as a viable memory device for word processing systems in U.S. Pat. No. 3,675,216, issued to Randell L. James. The James invention included a self-clocked recirculating shift register which was accessed through the detection of a flag code as the flag code passed the output window of the shift register. The flag code, therefore, was used to define the operating point in the shift register and data was accessed from the shift register at a rate of one character per revolution as the flag passed the output window of the shift register.
In order to change the operating point to some other location in the shift register it is necessary to move the flag to the desired location. When the operating point was changed in the prior art shift register based text processing system, the operation point or flag was moved one position per shift register revolution, or it was jumped via one shift register revolution to a location defined by a decode of the memory contents, for example, a line end or paragraph boundary code stored in the memory. The operation for relocating the operating point to a paragraph boundary is taught in U.S. Pat. No. 3,911,407 issued Oct. 7, 1975 to John Charlie Greek, et al and entitled "Text Processing System." In the Greek, et al, patent at column 13, lines 43-67 and column 14, lines 1-9 the operation flag is advanced in the shift register memory to a paragraph boundary by removing the flag from the data stream and holding it until double carrier return codes are decoded indicating the end of a paragraph. The operation flag is then reinserted into the memory stream.
Text processing operations which required the movement of the operation flag to a point not sufficiently defined by memory content were time consuming since the flag could only be moved one position per memory revolution. This time consumption becomes intolerable in a processor based system where many milliseconds of time can be lost while the processor waits for the flag to be advanced to a new operating point. An alternative solution to the problem of changing the operating point in a recirculating shift register memory has combined movement of the flag based on memory content decode with single moves in order to arrive at the desired memory location. But this solution requires the use of a complex algorithm and can still consume excess amounts of valuable time depending on the number of single moves that must be made to arrive at the desired location in memory.